When a voltage is applied to an organic electroluminescence device (hereinafter, occasionally referred to as an organic EL device), holes are injected from an anode into an emitting layer and electrons are injected from a cathode into the emitting layer. The injected electrons and holes are recombined in an emitting layer to form excitons. According to the electron spin statistics theory, singlet excitons and triplet excitons are generated at a ratio of 25%:75%.
A fluorescent organic EL device, which uses emission caused by singlet excitons, is inferred to exhibit an internal quantum efficiency of 25% at a maximum. Although having been used in full-color displays of a mobile phone, TV and the like, the fluorescent
EL device is required to use triplet excitons in addition to singlet excitons to further enhance efficiency.
In view of the above, a highly efficient fluorescent organic EL device using delayed fluorescence has been studied.
For instance, a thermally activated delayed fluorescence (TADF) mechanism has been studied. The TADF mechanism utilizes a phenomenon in which inverse intersystem crossing from triplet excitons to singlet excitons is thermally generated by using a material having a small energy gap (ΔST) between the singlet level and the triplet level. Thermally activated delayed fluorescence is described, for instance, in “Yuki Hando-tai no Debaisu Bussei (Device Physics of Organic Semiconductors)” edited by Chihaya Adachi, published Mar. 22, 2012 by Kodansha Company Ltd, pages 261 to 262.” For instance, Patent Literature 1 and non-Patent Literature 1 disclose organic EL devices using the TADF mechanism.
However, further improvement in luminous efficiency of the organic EL device in a high current density region is still desired.